Goal of the new tool would be:

to allow a User to update an existing geometry field that contains a Z Value
by searching for the nearest POINTZ (or POINTZM) of a Dem-Database

an existing Database (Source) Geometry field has been converted to a Dimension Z (or ZM)

the TABLE must be a SpatialTable, since ROWID will be used to update
An Dem-Database (Dem) exists that contains a POINTZ or POINTZM Geometry
The (Dem) contains a SpatialIndex
Both Geometries contain a valid Srid

Path to Database
Table
Geometry-Column

resolution of Dem
which by default can be calculated by

(extent_maxx_dem-extent_minx_dem)*(extent_maxy_dem-extent_miny_dem)/(double)rows_count_dem

assuming that the Dem has been completely filled with a Geometry
this parameter can be used to override this value if not correct.

sniff
which would allow the User to prepare the command syntax
receiving a message that all preconditions are fulfilled.
Source and Dem can be done separately or together
Goal is to ease the creation of the parameters and receiving information about what is to happen

update
everything done during a sniff, will be done here
if both Source and Dem are correct, the update will start
All of the (not NULL) geometries will be selected
if the Srid of the two are different, a transformed geometry will also be created
A function called gaiaUpdateZ (based on gg_transform.c gaiaTransformCommon) is called with both geometries and the extent of Dem
only Geometries completely contained inside the Dem will be processed (returning NULL)
a Geometry will be returned when any of the Z-Values (that are not 0.0) have changed (otherwise returning NULL)
if the result gaiaUpdateZ returns NOT NULL, the Geometry will be updated

gaiaUpdateZ will collect and send (when needed the transformed) points to retrieve_dem_points

The set resolution is used with ST_Buffer for the search_frame

but also to inform the user the amount of geometries/points and how many of those have been changed

usage: spatialite_updatez ARGLIST
==============================================================
-h or --help                    print this help message
========================== Parameters ========================
  -- -- ---------------- Dem-Data Database ---------------- --
-ddem or --dem-path  pathname to the SpatiaLite Dem DB 
-tdem or --table-dem table_name [SpatialTable or SpatialView]
-gdem or --geometry-dem-column col_name the Geometry column
	 must be a POINT Z or a POINT ZM type
-rdem or --dem-resolution of the dem points while searching
	 the automatic resolution calculation is based on the row_count
	 within the extent, which may not be correct!
	 Use '-rdem' to set a realistic value

  -- -- ----------------- Source Database ----------------- --
-d or --db-path pathname to the SpatiaLite DB
-t or --table table_name,  must be a SpatialTable
-g or --geometry-column the Geometry column to update
	 must  be a Z or a ZM Dimension type
	 use CastToXYZ(geom) or CastToXYZM(geom) to convert 
  -- -- --------------- General Parameters ---------------- --
-mdem or --copy-m [0=no, 1= yes [default] if exists]

  -- -- -------------------- Notes:  ---------------------- --
-I-> the Z value will be copied from the nearest point found
-I-> the Srid of the source Geometry and the Dem-POINT can be different

=========================== Commands ===========================
-sniff   [default] analyse settings without UPDATE of z-values 
-update Perform UPDATE of z-values 

Valgrind reports only the 2 well known GEOS leaks

sort -k2 -n -k1 3902_581820.txt -o 3902_581820.dhhn92.txt

.mode csv
.separator " "
.import 2007.berlin.dhh92.xyz berlin_dhh92_import

contains 42716 LINESTRING's, many of which are outside the Dem area (that contain geometries)

SQLite version: 3.17.0
SpatiaLite version: 4.5.0-devel

Source: srid 3068
Source: extent min x/y(3584.6332144,1452.2834924)
	       mix x/y(48941.5043681,37058.9517384)
Source: rows_count(soldner_segment) 42720
Source: geometry_type(1002) has_z[1]
Source: spatial_index_enabled[1]
Source 'berlin_admin_geometries.db'
will convert TABLE[berlin_street_segments] with GEOMETRY-Column[soldner_segment] Z-Value
	from nearest POINT found in
Source Database: has passed all checks.

Dem: srid 25833
Dem: extent min x/y(385000.0000000,5814000.0000000)
	    max x/y(397000.0000000,5826000.0000000)
Dem: rows_count(utm_point) 1116000000
Dem: resolution(utm_point) 0.1290323
Dem: geometry_type(1001) has_z[1]
Dem: spatial_index_enabled[1]
Source 'berlin_admin_geometries.db'
will convert TABLE[berlin_street_segments] or GEOMETRY-Column[soldner_segment] Z-Value
	from nearest POINT found in
Dem 'berlin_dhh92.db'
 TABLE[berlin_dhh92_2007] with GEOMETRY-Column[utm_point]
-W-> -rdem was  set. Using; resolution(0.9990000), overriding the calculated value: 0.1290323
Dem srid[25833]: is different from the Source srid[3068].
	 When searching for the nearest point, the Source points will be transformed to srid[25833].
Dem Database: has passed all checks.
	DB-file successfully converted !!!

-I-> geometries total[42716] changed[11808] ; points total[41010] changed z[41010] changed m[0]

SRID=3068;LINESTRING(24910.98653436587 21463.5093835775 35.39999999999999,24948.09927146823 21394.95263390163 35.60999999999999)

-I-> geometries total[42716] changed[0] ; points total[41010] changed z[0] changed m[0]

a (more or less) swift update can be made from the Dem-Database.

together with a config-file where the Dem information is stored,
could return the Z-value from a given srid,x,y position (or EWKT).

implemented in spatialite-tools-5.0.0 as spatialite_dem